This invention relates to the field of mechanical pulmonary ventilation. In particular, the present invention relates to a method and apparatus for pulmonary ventilation support which represents an improvement in the mode of mechanical ventilation known as "pressure-limited, time-cycled" ventilation.
Pressure-limited, time-cycled mechanical ventilation is well known in the art. Briefly described, in this mode of ventilation, a pressure limit is applied, during the inspiratory phase of respiration, to limit the peak inspiratory pressure to a selected (adjustable) maximum value. Once this maximum pressure value is reached, airway pressure is held at that value for the remaining duration of the selected (adjustable) inspiratory time period, during which gas continues to flow to the patient, albeit at a decelerating flow rate as the lungs fill. At the end of the selected inspiratory time period, the exhalation valve of the ventilator is opened for a selected (adjustable) expiratory time period, allowing exhalation either to ambient pressure, or to a positive end expiratory pressure ("PEEP").
Pressure-limited, time-cycled ventilation is distinct from volume-cycled ventilation, in which the inspiratory flow is stopped after a preselected tidal volume is delivered (to the patient and to the patient circuit), independently of peak inspiratory pressure, inspiratory time, and inspiratory flow rate.
Pressure-limited, time-cycled ventilation is often used for infants and neonates, particularly patients with hyaline membrane disease. An advantage of this mode of ventilation is that the pressure limitation feature reduces the risk of barotrauma.
The treatment of neonates with hyaline membrane disease and other respiratory disorders attributed to incomplete pulmonary development usually includes the administration of surfactants to the patient's lungs. The use of a surfactant typically results in an increase in the compliance of the lungs, resulting in greater tidal volume delivered to the lungs at any given peak inspiratory pressure. Consequently, to prevent over-expansion of the lungs (pulmonary hyperdistention), the peak inspiratory pressure limit must be periodically lowered as the surfactant takes effect. Recently, new surfactants and surfactant delivery methods have become available that produce dramatically more rapid and pronounced increases in compliance than have heretofore been possible, thereby requiring more rapid and frequent adjustments of the peak inspiratory pressure limit. Failure to adjust this limit properly could result in the delivering of a larger tidal volume to the patient, at a selected pressure limit, than the patient can bear without injury.
Mechanical ventilators in the prior art have not adequately addressed the above-described problems associated with changes in lung compliance, and therefore they have not obviated the need for frequent monitoring of tidal volume and adjustment of the pressure limit to compensate for these changes.
One approach to solving the above-described problems would be to provide a pressure-limited, time-cycled ventilator with a volume-cycling capability. The prior art, however, has viewed time-cycled, pressure-limited ventilation and volume-cycled ventilation as alternative ventilatory support modes, to be used separately, and not together in what may be termed a complementary sense. In other words, while the prior art has recognized the respective benefits of these two modes independently applied, it has not recognized any need for, or benefit of, employing a ventilatory support mode in which any given machine-delivered inspiratory breath can be both pressure-limited and either time-cycled or volume-cycled.
A survey of the relevant prior art reveals the following examples:
U.S. Pat. No. 3,523,527--Foster discloses a time-cycled ventilator which delivers an inspiratory breath for a preselected inspiratory time period, unless, before the expiration of the time period, a preselected pressure limit or a preselected volume limit is reached. In other words, the inspiratory flow is stopped whenever the first of three parametric limits (time, pressure, or volume) is reached; for any given breath, the ventilator may therefore be time-cycled, pressure-cycled, or volume-cycled. The ventilator is not pressure-limited, in that the inspiratory phase is not continued at the maximum pressure until the inspiratory time period has elapsed; the inspiratory phase is simply terminated.
U.S. Pat. No. 3,633,576--Gorsuch describes a volume-cycled ventilator that employs a time-cycled, pressure-limited ventilator as a pressurized gas source.
U.S. Pat. No. 3,729,000--Bell discloses a volume-cycled ventilator that maintains a constant delivered volume in the face of changing system compliance.
U.S. Pat. No. 3,961,627--Ernst et al. discloses a ventilator with pressure regulation during a first part of the inspiratory period, followed by flow regulation during a second part of the inspiratory period. A pressure limit and a flow rate limit are thus applied sequentially within an inspiratory time period, with the object of delivering the desired volume at the end of the inspiratory period.
U.S. Pat. No. 3,972,327--Ernst et al. discloses a volume-cycled ventilator with a pressure-limiting override feature. The ventilator can switch from flow rate-regulated inspiratory flow to a pressure-limited flow if, during inspiration, the buccal pressure, or the rate of increase of buccal pressure, exceeds preselected limits.
U.S. Pat. No. 5,129,390--Chopin et al. discloses a volume-cycled ventilator that establishes an optimum minute volume, with a set limit for proximal pressure ("airway passage pressure").
It will thus be appreciated that none of the above-referenced prior art patents suggests a time-cycled, pressure-limited ventilator with a volume cycle override that can be applied simultaneously with the pressure limit, to provide a machine-delivered inspiratory breath that is both pressure-limited and either time-cycled or volume-cycled.